1. Field of the Invention
The invention generally relates to optical disc recording or playing systems and, more particularly, to a method of and a system for modulating a 4-bit data sequence into a 6-bit code stream based on (1, 7) run-length-limited (RLL) constraints for digital data recording on an optical disc and to a demodulation method and system adapted for the modulation method and system.
2. Description of the Prior Art
In recording on the optical disc or the magnetic disc, a channel coding or modulation scheme known as (1, 7) run-length-limited (RLL) constraints is widely used in view of optical transmission characteristics in recording and reading and physical restrictions involved in pit formation. In the channel coding, for the sake of servo control, the digital sum value (DSV) is controlled so as to satisfy a constraint on low-frequency components as is well known in the art. However, conventional (1, 7) RLL scheme can not afford a satisfactory control of the DSV to some bit patterns to fail in suppression of DC components, resulting in, for example, information signal components mixing in the servo signal band. This presumably causes a problem of adversely affecting the servo performance.
From this point of view, Japanese unexamined patent publication No. 06195887 (1994) discloses "Recording Sign Modulating Device," in which a suggestion is provided to reduce a DC component by avoiding repetition of a specific bit pattern.
Japanese unexamined patent publication No. 10340543 (1998) discloses "Encoding Device, Decoding Device, Encoding Method and Decoding Method Therefore," in which a suggestion is provided to reduce a DC component by inserting a DSV control code by a number of bits having a redundancy as less as possible.
Though the publication No. 06195887 can reduce repetition of a specific bit pattern through bit inversion and/or making patterns random but can hardly suppress a DC component to a satisfactory extent. The publication No. 10340543 seems to provide a preferable DC suppression result but at the same time lowers the storage capacity due to insertion of DSV control codes.
It is therefore an object of the invention to provide a four-to-six modulation code system (or table), and a four-to-six modulation method and system that uses the modulation code table to provides a 6-bit code stream satisfying the (1, 7) RLL and DSV constraints without the need of merging bit or DSV control bit.